This invention relates to telephone security systems and more particularly to detectors for "Black Box" toll fraud. "Black Box" is a term applied to a contrivance attached to the telephone lines of a party who receives toll calls for the purpose of preventing telephone company equipment from initiating billing of the originating call. In general, the "Black Box" prevents the telephone circuit from returning answer (off-hook) supervision to the calling station by a resistance network applied to the phone at the called station. Conversation between the parties is generally conducted over the resistance of the device.
At present there are two known methods of observing a "Black Box" call. The first method is human observation of the supervisory conditions of the equipment and continuous monitoring of the suspected equipment for a fraudulent call. This method requires the full time availability of a person to serve as the monitor. It requires the person to observe all good calls as well as fraudulent calls in order to identify and determine that a "Black Box" is being used. The need for such thoroughness is dictated by legal requirements of the data as evidence in the prosecution of offenders.
The second method employed in the past is to bridge the line of the suspected subscriber with a device that attempts to sense a line change of the subscriber phone, and monitoring that line for voice signals and incorrect supervision. The difficulty with this technique is that existing equipment available to perform it is expensive and unreliable. The bridge-type equipment reacts to voice talk-off, i.e., false operation, causing the equipment to register a fraudulent call incorrectly.
There is need for an automatic unmanned reliable toll fraud detector--one which registers only fraudulent calls--in order to provide completely credible information useful as legally qualified evidence in the prosecution of perpetrators of such calls. This invention is directed toward the provision of such a detector.